The problem of controlling read/write head positioning over memory tracks in a disk drive has seen much attention in the prior art. Control systems have been devised to position a selected head over a track even when disturbing forces are present. Bearing and shaft imperfections cause attached disks to wobble when spinning. Such wobble is called run-out, and prior art control systems have been developed to position a head to follow changes in track position even in the presence of such run-out.
Besides run-out, other parameter variations render the position control problem a difficult one to solve. Gain variations, changes in resonance amplitudes and frequencies and other perturbations in a disk actuating control system may significantly change over time. Such parameters also vary from disk drive to disk drive. For instance, the gain of a disk drive control system can range over large margins due to temperature, aging, and unit-to-unit variations. Such changes can cause a head's "settling time" to increase substantially and thus degrade the performance of a disk drive. Further, if system gain falls, disturbance rejection and the ability to follow run-out are both reduced.
The prior art is replete with head/arm actuator control systems In U.S. Pat. No. 4,135,217 to Jacques et al., a servo system provides both coarse and fine positioning. During coarse positioning, stored repeatable error information and offset information is used to generate error signals to control the movement of the head/arm actuator. After coarse positioning, fine positioning information, including offset and stored repeatable error information are used to generate fine positioning error signals to control movement of the head/arm actuator. In U.S. Pat. No. 4,204,234 to Noble, track following is achieved by first storing sensed data signals. Those signals are then compared with subsequent sampled signals, and the differences are used for control purposes.
Alaimo et al. in U.S. Pat. No. 4,630,190, use a reference track to enable calculation of position correction signals from errors which occur as a result of disk wobble. Those correction values are then used to compensate for run-out during read/write operations.
In U.S. Pat. No. 4,412,165 to Case et al., position error signals are derived from a head that is constrained in a correct, on-track position. The position error signals are then stored and employed as a datum against which subsequently read signals are compared to provide further error signals for head/arm position correction.
Sidman, in U.S. Pat. No. 4,536,809, employs a training time in which signals are sensed from a servo track, to enable the generation of a set of misposition error signals. A digital filter adjusts the phase of the digitized misposition signals to compensate for known servo control system and low-pass filter lag. It also adjusts phase lead and gain terms of the fundamental frequency and selected harmonics to enable rejection of high frequency harmonics. Thereafter, the filter generates correction signals from the phase-corrected misposition error signals, with the system iterating the process for one or more disk rotations to obtain further refined misposition error signals. That information is then stored and used with subsequently accessed data, to enable improved track following by the read/write head.
Additional servo position correction and servo track compensation systems can be found in the following IBM Technical Disclosure Bulletins: Griffiths et al., "Self-Calibrating Disk Storage Apparatus", Volume 19, No. 6, November 1976, pp.1991, 1992; Betts, "Null Servo Pattern", Volume 18, No. 8, January 1976, pp. 2656, 2657; Palmer et al., "Packwriter Write Correction System", Volume 13, No. 11, April 1971, page 3505; Matla et al., "Track Servo System Compensating Pattern Defects", Volume 22, No. 8A, January 1980, pp. 3269,3270; Santana, "Generation of Position Correction Signals for All Disk Pack Surfaces", Volume 12, No. 11, April 1970, page 1891; Brock et al., "Recording Position Signals on Record Disks", Volume 22, No. 8A, January 1980; and Brock et. al., "Detecting Erroneous Servo in Record Storage Apparatus", Volume 21, No. 3, August 1978, pp. 932, 933.
Notwithstandinq the many servo read/write head control systems in the prior art, parameter variations that occur due to aging, temperature and environmental disturbances still present problems with respect to head positioning. As indicated above, modest gain variations can cause substantial changes in the settling time of a head and track positioning performance. A real time servo control system for correction of such parameter variations is needed.
Accordingly, it is an object of this invention to provide a control system for a disk drive actuator which is adaptive in response to parameter variations.
It is another object of this invention to provide an adaptive control system for a disk drive actuator that enables rapid and efficient alteration of servo system parameters during real time operation.